East Okement Farm DCNN8825 – A harsh but moderated environment.

50.70349 -3.97754 Met Office CIMO Assessed Class 4 Installed 1/1/1991 Elevation 408 metres amsl.

East Okement Farm is one of the highest in Devon on the edge of Dartmoor. The weather station was installed to replace the original Okehampton weather station site at the nearby military camp and firing range which was closed down in 1990. The former site at 370 metres amsl was a somewhat harsh environment, the new site even more so. It certainly takes dedication to the cause to farm such sites which is amply displayed by the “Commoners” operating this site.

In situations like this I always personally like to distance any criticism of those operating the site from my remarks concerning the Met Office who are the responsible body. I have the greatest respect for the farm owners and observers who fulfil a vital function. My complaints lie with the Met Office who attempt to reconstruct an ongoing national temperature record from sites that were never originally intended for long term climate reporting rather just local conditions.

The issue here is not that the site (if correctly positioned) is unlikely to be representative of a wider area because it certainly could be. The area of Dartmoor National Park is marginally under 1,000 square kilometres (or put another way about 0.8% of the area of England) so it makes perfect sense to measure the climate of such a large area. It is rather the detail of the siting that potentially causes the instrumentation to not accurately reflect the wider area as detailed below.

The farm is obviously going to be located in the most sheltered spot available and will over time have been deliberately sheltered from the worst effects of the harsh weather, most notably almost continuous wind. The farm is located midway between the nearby tor at 480 metres and the East Okement River which breaks the 375 metre contour at this point. A considerable slope but the immediate surroundings of the farm seem acceptably flat for a screen location.

To the north and east, sheltering belts of trees break the worst of winds from those directions. The principal exposure is to the south and west though topography to those largely prevailing wind directions will moderate the wind speed. It is the immediate screen surroundings that dictate this was rated by the Met Office as Class 4 and its attendant inaccuracy. The 3 metre radius circle just makes it within the enclosed area and will be the basis for the Met Office judgment.

this reflects an issue where I feel the Met Office is remarkably inconsistent. Bradford Lister Park is rated as Class 5 with this below what a 30 metre radius looks like. In fact the 3 metre radius does not reach the footpath.

I find it remarkably difficult to understand why the Met Office rates East Okement Farm as significantly superior to the Bradford site. When looked at even deeper the issue gets worse, The Farm location is surrounded by dry stone walling not an open railing fence as at Bradford. Dry stone walling is a very common feature in upland Devon and is built to typical local standards.

“A standard, freestanding wall usually stands 1.4m (4ft 6in) above ground level. Boundary walls (perimeter walls to farms, estates, etc) are more commonly 1.6m (5ft 3in).”

The screen in East okement Farm is surrounded by such walls which in all likelihood will be of the higher 1.6 metre perimeter standard. The image in the “Commoners” link above shows the owners in front of such walls of the greater height. This will mean the screen itself is sitting within a wind sheltered unnatural enclave.

Under most weather conditions, in such otherwise open countryside , this would historically have not posed a major problem. Traditional Liquid in Glass Thermometers are slow to respond especially in the Stevenson screen environment. However, on the 22nd September 2017 in accordance with a nationwide changeover by the Met Office, the maximum reading LIGT (only) was replaced with a fast reacting Platinum Resistance Thermometer (PRT). Despite the notable feature of the electronic PRT and its “data logger” memory recording ALL temperatures by the averaged 60 second period for ALL day making any other air temperature reading instruments unnecessary, the minimum LIGT was always retained at manual sites and used for those recordings in preference to the PRT.

As highlighted in my report on meteorological averaging, the unusual derivation of “means” is solely dependent on points of extreme, small changes in these can cause major end differences. In certain calmer weather conditions this enclosed site will result in stagnant air developing within the screen warming above the outside ambient temperature. Even brief effects will be recorded by the PRT for maximum readings but there will be no compensatory lower minima at any time from the LIGT. A simple recipe for increasing both maxima and more so means.

In conclusion whilst this site is well observed and maintained by its custodians, it is still a poor site which probably should be rated Class 5. It is not suitable for long term climate reporting.

Source: https://tallbloke.wordpress.com/2025/08/12/east-okement-farm-dcnn8825-a-harsh-but-moderated-environment/